Surrogate ratio methodology for the indirect determination of neutron capture cross sections

Abstract

The relative $\ensuremath{\gamma}$-decay probabilities of the $^{162}\mathrm{Dy}$ to $^{161}\mathrm{Dy}$ and $^{162}\mathrm{Dy}$ to $^{164}\mathrm{Dy}$ residual nuclei, produced using light-ion-induced direct reactions, were measured as a function of excitation energy using the CACTUS array at the Oslo Cyclotron Laboratory. The external surrogate ratio method (SRM) was used to convert these relative $\ensuremath{\gamma}$-decay probabilities into the $^{161}\mathrm{Dy}$($n$,$\ensuremath{\gamma}$) cross section in an equivalent neutron energy range of 130--560 keV. The directly measured $^{161}\mathrm{Dy}$($n$,$\ensuremath{\gamma}$) cross section, obtained from the Evaluated Nuclear Data Files (ENDF/B-VII.0), was compared to the experimentally determined surrogate $^{161}\mathrm{Dy}$($n$,$\ensuremath{\gamma}$) cross section obtained using compound-nucleus pairs with both similar ($^{162}\mathrm{Dy}$ to $^{164}\mathrm{Dy}$) and dissimilar ($^{162}\mathrm{Dy}$ to $^{161}\mathrm{Dy}$) nuclear structures. A $\ensuremath{\gamma}$-ray energy threshold was identified, based upon pairing gap parameters, that provides a first-order correction to the statistical $\ensuremath{\gamma}$-ray tagging approach and improves the agreement between the surrogate cross-section data and the evaluated result.